1. it’s quite easy to make a map like mine, if you’re able to code a little Javascript
2. my approach (polylines) doesn’t scale very well – you can put a dozen lines up like this, but don’t plan on doing a complete urban bus network; it’ll be way too slow. A tiled raster layer is the best way to go for a complicated network, although user interaction would be tricky.
3. client updates would also be tricky, although there might be a way to use Google’s existing “My Maps” capability to let them build up a KML polyline or two, and then import that KML into a website.
4. no experience with the smartphones; can’t say much there

That said, you can usually think up a scenario that defeats it at some level – here’s mine. (I think this is just a data gap – some of the TTC routes actually keep running at times not shown on the published schedules.)

While I’m at it, I should also give some praise to Ian Stevens’ transit map, which is great for seeing Toronto bus routes. Simple but highly effective.